Process for treating material in plasma environment

ABSTRACT

An improved method of fabricating metal-semiconductor interfaces such as Schottky barriers and ohmic contacts. There is disclosed apparatus and method (or process) for chemically converting, etching, or passivating the surface of a material, such as the surface of a silicon wafer, in a gaseous plasma environment consisting of atomic, neutral nitrogen which causes the surface of the material to be resistant to otherwise subsequent nascent surface oxide buildup. This process is particularly useful in manufacture of Schottky diodes, transistors, and other electronic components or discrete and integrated devices requiring high quality metal-semiconductor junctions or interfaces.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a plasma process for treatingmaterial, and more particularly to a process for fabricating Schottkybarriers and ohmic contacts in devices normally employed within thesemiconductor electronics industry.

2. Description of Prior Art

Examples of patents which relate to formation of Schottky barriers byuse of a plasma are U.S. Pat. No. 3,795,557, issued Mar. 5, 1974 to A.Jacob, and U.S. Pat. No. 3,879,597 issued Apr. 22, 1975 to Bersin et al;the disclosures of these two patents are incorporated herein byreference.

Metal-semiconductor rectifying devices have been investigated since thelate 1800's. Sometime thereafter, around the turn of the century, apoint-contact rectifier (metal-semiconductor junction device) may havefound practical utility as evidenced by U.S. Pat. No. 775,840 issued in1904 to J. C. Bose. In the late 1930's W. Schottky continued to studythe metal-semiconductor potential barrier, and the suggestion that thispotential barrier could arise from arrangement of space charges withinthe semiconductor, not necessarily requiring presence of an adjacentchemical layer, is attributed to him. This development continued throughmore recent history, and the metal-semiconductor junction rectifierpresently employed in electronic circuit applications is commonly calleda Schottky diode or Schottky transistor.

The Schottky barrier contact or interface, as noted, is a rectifyingmetal-semiconductor junction. Such Schottky barrier contacts utilize theSchottky effect based upon rectification characteristics exhibited bywell known metal-semiconductor interfaces. Generally, the electricalcharacteristics of these contacts depend upon what is termed the "workfunction" of the metal, as well as the electron affinity in thesemiconductor material. The work function is generally defined as theminimum energy necessary for an electron to have in order to escape intovacuum from initial energy at the Fermi level, in a metal-vacuum system.In other words, in a metal, the Fermi level is an energy reference levelfrom which an electron is removed to the free state (vacuum electron) byan amount of energy equal to the work function.

High frequency response of these Schottky contacts or diodes is good,the positive results flowing from conduction phenomena which occursunder forward bias, caused primarily by majority carriers falling fromthe semiconductor into the metal. Accordingly, the otherwisefrequency-limiting effect of minority carrier storage tends to beminimized. High frequency response of Schottky barrier diodes makes themquite useful in various high frequency applications, such as high speedlogic and memory circuits, microwave applications, etc. The foregoing ispresented as background in the history and application of the resultingsemiconductor device, the device itself being prepared by thechemical/physical process of the present invention, the background ofwhich follows.

In most, if not all, types of integrated circuits and discrete devices,it has been usually necessary to make ohmic contacts to various regions.For example, in Schottky-clamped-integrated circuits, and thoseemploying Schottky barriers as active devices, whether integrated or indiscrete form, it had been generally necessary not only to make ohmiccontacts to several regions, but also to fabricate good Schottky barriercontacts. It is generally preferred to use the same metallization forboth ohmic contacts and Schottky barriers in a particular device.Although one is not limited to this constraint, it is generallypreferred from a production point of view.

In the prior art or conventional technology, when aluminum and/or itsalloys with silicon and copper are used for metallization, a problem ofnascent oxide (oxide which comes into being, or forms, or develops overbare silicon regions) is solved to some extent by heat treatments in aneutral ambient environment at temperatures between 450° C. and 550° C.Under appropriate processing conditions, aluminum atoms can move throughthe nascent oxide layer and make intimate contact with the siliconregions, yielding good Schottky barriers and ohmic contacts. Metalsother than aluminum can yield poor results.

If a metal-silicide, such as platinum-silicide, palladium-silicide orrhodium silicide, is used to form the Schottky barriers and ohmiccontacts, the nascent oxide layer on the silicon substrate can causeproblems. The platinum-silicide is usually preferred over the othersbecause its barrier is the largest on N-type silicon and because of itssuperior reliability/performance characteristics. In such metallizationschemes other metal layers like titanium-platinum-gold (beam leadtechnology) and titanium-tungsten-aluminum are deposited on thesemetal-silicide layers. These metal-silicide devices are more reliablethan those devices using aluminum and its alloys only.

In prior art platinum-silicide processes, the nascent oxide on siliconregions is inevitably present during the processing of the siliconwafers. One prior art solution was to use chemical solution ofhydrofluoric acid to etch away oxide on the silicon surface; however, assoon as the wafers are rinsed in water, an oxide layer is againimmediately formed. Thickness of this nascent oxide layer is about 25Angstroms, (one Angstrom=10⁻¹⁰ meters) or greater, depending onprocessing conditions. This thickness is enough to prevent platinum fromreacting with silicon to form a good, uniform layer of platinum silicidewhen the wafers are heat treated in a neutral ambient, even at 650° C.This is a problem of the prior art.

Another prior art or presently used process to remove nascent oxide fromsilicon regions where platinum-silicide ohmic contacts or Schottkybarriers are to be formed, is to do in-situ sputter-etching of thesilicon wafers prior to platinum deposition in the same sputteringsystem. Sputter-etching is basically an ionic bombardment of the surfaceto be cleaned off. After the usual ambient pressure pump-downs in thesputtering system, back-filling with an inert gas like dry argon to apressure of about 20-40 microns, and applying radio frequency (ornegative d.c.) high voltage to the electrode on which the silicon wafersare placed, an argon plasma is obtained. Depending on thevoltage/wattage of the d.c./r.f. power, and its duration, a certainthickness of oxide, silicon and/or other substances present on the wafersurface are removed. Sputter-etching, in this case, is a bombardment ofthe surface of silicon with highly energized ions of argon, thusremoving these impurities from the surface of the silicon. Reference toU.S. Pat. Nos. 3,737,743 and 3,855,612 will provide further informationon sputter-etching, as it relates to Schottky barrier devices.

The thickness of silicon oxide expected to be removed in thissputter-etching step is in excess of 100 Angstroms. Usually, moreetching than that which is necessary is practiced in the processing toassure complete removal of the nascent oxide. This is done because anincomplete removal of the nascent oxide gives patchy or poor silicideformation, which in turn results in bad ohmic contacts and bad Schottkybarriers.

However, there are problems also associated with this sputter-etchingprior art process. If the sputter-etching is insufficient to remove allnascent oxide contaminants, there are problems with ohmic contacts andSchottky barriers since this imperfect process permits random variationsuch as increases in resistive values of the ohmic contacts. In turn,this permits other problems such as voltage and power loss at theseincreased-resistivity ohmic contacts with accompanying unpredictabledevice performance and reliability problems. Further, random variationsuch as decreases of reistivity of Schottky barrier potential heightspermits higher leakage currents in Schottky devices as well ashigh-field edge-effects, with accompanying unpredictable deviceperformance and again reliability problems. Finally, for this situationof insufficient sputter-etching, a problem of lifting of beam leads andcontacts from the ohmic contact and Schottky barrier regions can occur.

But, suppose the sputter-etching were excessive, rather than inadequate.If excessive sputter-etching of the silicon wafers is done to ensurecomplete removal of nascent oxide layer, and to ensure good platinumsilicide formation then other problems can occur as follows: First,excessive sputter-etching can remove an inordinate large amount ofsilicon oxide, in particular the phosphorus doped silicon oxide normallycovering the emitter and various cross-under n-type regions. If thissputter-etching is not controlled properly, then the thin emitter glass(silicon) gives rise to low breakdown voltage and higher leakage andshorting problems. Next, if the oxide films have pinholes, excessivesputter-etching enlarges these pinholes, and can cause a short circuit.

Yet another problem which can occur during the prior art sputter-etchingprocess is that if water vapor is present in the sputtering system, thenan oxide layer can be created in this cycle due to reactive sputtering.Therefore, instead of removing the nascent oxide layer from siliconregions, an additional oxide layer is created- The water vapor in thesystem can come from inappropriate drying of the wafers prior to loadingthem into the sputter-etching system, and/or it can be obtained fromother sources like argon gas, vacuum system and jigs used in the system.The mechanism of the formation of an oxide layer in this step is due tothe reaction of the oxygen and/or hydroxide ion in the prior art ionizedargon plasma with silicon. Therefore, instead of improving the siliconsurface to give a good ohmic contact and a good Schottky barrier, thesilicon surface is oxidized further, yielding bad results.

The present invention provides a solution to this multiplicity ofproblems of the prior art. The present invention employs and provides animproved process for manufacture of metal-semiconductor interfacesincluding but not limited to platinum-silicide interfaces, Schottkybarrier potential junctions, and ohmic contacts, which improved processeliminates the sputter-etching step and thereby eliminates all of theabove-detailed problems associated with the presently used and prior artsputter-etching technique. The present invention employs a novel plasmaprocess to be described in detail hereinbelow.

SUMMARY OF THE INVENTION

The present invention relates to a process for treating material in aplasma environment. More particularly, the present invention relates tochemically converting, etching or passivating the surface of a materialin a plasma environment. These processes include the step of exposingthe material to gaseous plasma consisting of atomic, neutral nitrogen.This exposure passivates the material's surface, the surface then beingfree from and resistant to further nascent oxide or nascent oxidebuildup.

Thus, in the semiconductor industry, where metal-semiconductor junctionsor interfaces are manufactured for research or commercial purposes, itis clearly advantageous to employ the processes described and claimedherein, since they eliminate problems associated with the prior artsputter-etching technique.

It is thus an object of the present invention to provide an improvedprocess for treating material in a plasma environment.

It is a further object of the present invention to employ a gaseousplasma consisting of atomic, neutral nitrogen to passivate the surfaceof a material where the treated surface is then free from further oxidebuildup.

Further objects and advantages of the present invention will be apparentafter reference to the detailed description of the preferred embodimentsand the appended drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a semiconductor substrate with anascent oxide layer;

FIG. 2 is the cross sectional view of FIG. 1 showing incomplete removalof the nascent oxide layer;

FIG. 3 is a cross sectional view of a semiconductor substrate showingresults of excessive sputter-etching;

FIG. 4 is a schematic flow diagram incorporating the process of thepresent invention and comparing same with prior art;

FIG. 5 is a graph showing performance characteristic results obtainedfrom a device constructed in accordance with principles of the presentinvention; and,

FIG. 6 is a cross sectional view of a semiconductor substrate showing nonascent oxide buildup, after treatment by the techniques of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, silicon substrate 101 interfaces with silicondioxide layer 102 at boundary 104. Nascent oxide layer 103 is shown tobe present on the silicon surface and can have a thickness ofapproximately 25 Angstroms. As noted earlier, nascent oxide on thesilicon region forms immediately at room temperature and it is thislayer which prevents the formation of good platinum silicide interfaces.

FIG. 2 depicts essentially the same cross section as shown in FIG. 1after treatment to remove nascent oxide. In this instance, incompleteremoval of nascent oxide yields patchy platinum silicide as shown atjunctions 203. Since all nascent oxide has not been removed, theresultant patchy platinum silicide results in a poor set of ohmiccontacts and poor Schottky barriers. This is the result of inadequatesputter-etching. Thus, usually more sputter-etching than less isemployed to ensure complete removal of the nascent oxide. However excesssputter-etching also gives poor results, as shown in FIG. 3.

Referring to FIG. 3 substrate 301 essentially corresponds to substrates201 and 101 of the earlier figures, and silicon dioxide layer 302essentially corresponds to its counter part layers 202 and 102 in theother figures. However, dotted line 303 indicates the outline of whatwould have been the silicon dioxide layer, had it not been removed byexcess etching under prior art conditions. It can be seen that asubstantial amount of silicon dioxide has been removed because ofexcessive sputter etching, to ensure removal of the nascent oxide layer.However as noted, this permits other problems to develop such as lowbreakdown voltage high leakage and shorting problems.

By contrast with the above described three figures, refer to FIG. 6wherein it can be seen that silicon substrate 601 is in contact withsilicon dioxide layer 602. The wafer had been subjected to the processof the present invention and thus shows the edge of a passivated surface603, free from nascent oxide buildup and ready to accept a layer ofplatinum or other metal to form a Schottky barrier or an ohmic contactas describd herein.

Referring to FIG. 4, a schematic diagram of process flows is depicted.Oxidizing and diffusing equipment 401 provides an oxidized wafer (notshown) of silicon with photo resisted areas. This wafer thus has etchedout areas down to the base substrate (silicon). Cleaning and dryingapparatus 402 operates upon the wafers from equipment 401. Thisequipment is standard in the industry and details thereof need not bepresented herein for full comprehension of the present invention.Descriptions of equipments 401 and 402 and their operations can be foundin such references as: G. E. Moore in "Microelectronics", E. Keonjian,ed., McGraw Hill Book Co., Inc., New York, (1963), p. 2.76.

In accordance with principles of the present invention, the oxidizedwafer is then presented to and subjected to a nitrogen plasma asdeveloped in plasma etching equipment 403. Plasma machine 403 is alsocommerically available, and its operation is described inincorporated-by-reference U.S. Pat. No. 3,879,597 entitled "PlasmaEtching Device and Process". (A plasma can be made by subjecting a gasat low pressure to radio frequency voltage. The etching itself isaccomplished by placing the gas at low pressure in a quartz cylindersurrounded by a source of radio frequency power, such as a coil or anumber of electrodes, and then energizing the coil or electrode withhigh voltage at radio frequency. The production of a plasma is indicatedby a bright glow within the quartz cylinder.)

After the proper length of time and temperature in plasma equipment 403,the surface of the wafer is passivated properly, whereafter it issubjected to platinum sputter-deposition (or other metalic deposition)in deposition equipment 404. Then the wafers subjected to otherannealing equipment 405 for processing the wafer into what will becomeuseful Schottky or ohmic contact devices. Annealing equipment 405 isstandard and further information can be obtained in G. E. Moore (ibid).

More specifically, the present invention eliminates the sputter-etchingstep described earlier (and performed in 403') and eliminatessputter-deposition of platinum (in 404') consequently eliminatingvarious problems discussed in connection with the prior art. The basicidea is to treat the silicon wafers in a nitrogen plasma consisting ofatomic, neutral nitrogen; the basic discovery is that such treatmentproperly passivates the silicon surface and prevents nascent oxidethereon. Atomic, neutral nitrogen is intended to mean not the molecularN₂ nitrogen, and not to mean ionic N+ nitrogen but simply atomicnitrogen uncharged and unbonded to other nitrogen atoms.

The conditions under which such atomic nitrogen can be obtained for usein the process of the present invention are RF power equal 300 watts,pressure equals 1 torr, duration of wafer exposure equals 5 minutes.When platinum is thereafter deposited on silicon wafers in equipment 404which have been treated in such nitrogen plasma, and subsequently heattreated in annealing apparatus 405 at 650° C., it has been observed thata good platinum silicide is formed on the silicon regions. The criterionof a good platinum silicide is that it should be opaque under infraredexamination, and the barrier height of of the platinum silicide Schottkydiode on n-type silicon should be 0.84±0.03 eV, (electron volts). Bothof these criteria were met on samples made according to the presentinvention.

Referring finally to FIG. 6, a logarithmic plot of forward current vs.forward voltage as regards devices processed in accordance withprinciples of the present invention is shown, from which performanceresults can be calculated. The logarithmic plot is linear thusindicating the logarithmic (or exponential) physical nature of operationof the device. From the slope of the line which was obtained frommeasurements on the device prepared in accordance with principles of thepresent invention, and from other data such as geometrical size of thedevice, the Schottky barrier for a metal-semiconductor interface orjunction has been calculated to be 0.82 eV. This falls within the properbarrier height range for a platinum-silicide Schottky diode, whichshould be 0.84±0.03 eV.

A theoretical explanation or model to account for the improved devicesflowing from this discovery and for this substantial advance in the artof semiconductor processing, is as follows. Atomic nitrogen, N,generated in a nitrogen plasma in an etch tunnel, and applied to asilicon wafer having nascent oxide on its surface, breaks the oxide.Such a plasma of atomic nitrogen is highly reactive and also breakshydrated oxide molecules in addition to breaking the oxide, and theyboth are removed from the surface. The dangling bonds of silicon atomson the surface latch onto nitrogen atoms providing a monolayer, or moreof nitrogen coverage. This arrangement of nitrogen atoms on the siliconsurface retards, or prevents, the interaction of oxygen and water vaporwith silicon at room temperature. Thus the formation of a nascent oxidelayer on the silicon surface is retarded and prevented.

Recapitulating, the present invention relates to treatment of siliconregions with a nitrogen plasma containing free radicals of nitrogenwhich react with the surface layer of the silicon oxide to remove theoxide from the silicon regions and thus passivate them so as to retardgrowth of the nascent oxide layer at room temperature. The siliconwafers so treated, can then be deposited with a metal like platinumwithout sputter etching; the platinum comes in intimate contact with thesilicon surface. Heat treatment at some appropriate temperature causesthe platinum to react with the silicon to form a monosilicide whichgives good Schottky barriers on silicon regions having moderateresistivity and good ohmic contacts on those having low resistivity.(The basic difference between forming a Schottky barrier or an ohmiccontact relates to the concentration of dopant in the substrate. Thenormal definition for the amount of concentration required to separate aSchottky barrier from an ohmic contact is: if the concentration ofdopant in the substrate is 5×10¹⁸ atoms per cubic centimeter or morethen the contact is ohmic; if less the contact is Schottky.)

This invention can be embodied in other forms without departing from thespirit or essential characteristics thereof. For example, if nitrogencan be obtained in the uncharged, atomic state by means other than thespecific plasma generating equipment disclosed and if that free nitrogenis used in the process described herein, that condition is intended tobe included within the purview of the present invention.

Thus, the present embodiments are to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed is:
 1. A process for passivating the surface of asemiconductor material in a plasma environment comprising the stepof:exposing the material to a gaseous plasma consisting of atomic,neutral nitrogen to obtain said passivated surface on said material,said surface being free from nascent oxide.
 2. A process as defined inclaim 1 and wherein said material comprises silicon wafers.